Charge Distribution and Chemical Bonds in UO3 Polymorphs
Résumé
We have employed atom-in-molecule (AIM) approach to study the charge distribution in UO3 polymorphs. The AIM method was originally used in theoretical chemistry to study charge transfers among atoms in molecules but it was quickly extended to crystalline solids. AIM analysis can be applied to experimental determinations of charge density obtained by sophisticated ultra-high-resolution X-ray diffraction data. One of the main difficulties in this case is to correct the experimental charge density for atomic nuclear motions. While in the case of light atoms the harmonic approximation of the atomic nuclear motions might be sufficient, this is generally not the case for heavier atoms where anharmonic effects can be significant, even at low temperature. A simultaneous robust description of the anharmonic displacements of the atomic core densities and of the deformation densities of the valence electrons is not a simple achievement because of the analytical coupling of the effect of these physically distinct origins. The unpleasant bias of this coupling shows up in unrealistic values of the charge deformations and, to an even stronger extent, in the Laplacian maps of the charge density. This is a strong motivation to address the problem of deformation densities and bond analysis in systems containing heavy atoms using quantum calculations of the charge density. In this respect, the case of Uranium (and actinide) containing systems is quite extreme, but their fundamental properties and technological applications are so important that gathering information about the characteristics of their chemical bonds can actually help to shed light on complex subjects related to nuclear energy production and to nuclear safety. We believe this can actually help to understand and design better fuels, and waste forms for the disposal of radioactive elements.
The local structure of uranium atoms differs greatly in each of the UO3 polymorphs, but the formal valence of uranium in all of them is formally +6. To asses the effect of the different uranium environments in a quantitative way in two polymorphs, we have applied AIM analysis to the electron charge density obtained by density functional theory (DFT) calculations for each of those relaxed structures. We have obtained robust estimates for Bader’s charges and the Hessian matrix at the critical points of the charge density, where the gradient of the charge density vanishes. Finally, we have compared the results obtained on -UO3 (U in an octahedral environment) and -UO3 (U in a pentagonal bipyramid environment). Finally, we have also analyzed a more complex system, La6UO12, where U atoms are in an octahedral environment: this allows us to study the charge transfers and the characteristics of the U bonds in a structure containing another element, and to address the problem of transferability of this analysis on simple oxides to more complex structures.